The present invention is generally directed to inductively coupled plasma (ICP) tubes and, more particularly, to apparatus for initiating plasma in ICP tubes.
High frequency induction plasma tubes are conventional devices for producing high temperature gaseous plasmas. Conventional ICP tubes include an electrical induction coil surrounding an enclosure which may include an internal shield. The coil is connected to a source of high voltage radio-frequency energy, conventionally in the frequency range of 400 kHz to 5 MHz. A low pressure gas, such as argon, is commonly used as the ionizable gas forming a hot gaseous plasma inside the shield assembly.
To ionize the gas and initiate the plasma, a high voltage is applied to the induction coil, which induces a sufficient voltage on the shield to establish an arc to a lower mounting flange beneath the shield. The arc initiates the plasma at very low pressures, i.e., around 30 microns. However, at higher pressures the arcing does not occur and the plasma is not established. An ICP tube using the induced shield voltage to initiate the plasma is described in Donald E. Hull et al., "Combination Induction Plasma Tube And Current Concentrator For Introducing A Sample Into A Plasma," U.S. Ser. No. 867,127, filed May 27, 1986, and incorporated by reference herein.
An alternate method for initiating plasma in an ICP tube is taught in Felix Dothan, "Triggering Device For A Vacuum Arc In A Plasma Centrifuge," U.S. Pat. No. 4,612,477, dated Sept. 16, 1986. As therein described, a spark gap is formed adjacent to the cathode. The resulting vacuum arc then creates the plasma in the device.
The prior art ICP devices typically require a very low pressure in the order of 20 to 50 microns for reliable plasma initiation. Such low pressures can be difficult to obtain. However, at pressures above about 50 microns the initiation of a plasma becomes uncertain and, at still higher pressures, not possible using induced voltage on the shield.
This problem is substantially minimized with the present invention and an improved system is provided for initiating a plasma at internal pressures above 50 microns.
It is an object of the present invention to initiate plasma in an ICP tube with internal pressures greater than about 50 microns.
It is a further object of the present invention to provide the improved starting characteristics with minimum change to existing ICP tube design.
Yet another object of the present invention is to enable plasma initiation without tube damage from arcing within the tube.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.